Usability tests often present some real surprises where the participant's mental model does not match what they're presented with.
What are the biggest or most interesting surprises you've come across?
What was maybe the biggest surprise to me in the first few usability tests I conducted, that people really blame themselves when they're unable to complete a task on an obviously bad UI.
And I still like to be reminded how little people know about some domains. For example, how little people care about banking or telecommunications, how little jargon they understand and care to learn.
After a long, and rather baffling navigation of an ecomms site, where the user had been tasked with attempting to purchase a product, I had to ask them to think aloud to try and get a handle on their behaviour. Turns out he was looking for the phone number to call the company and buy the product over the phone!
It always amazes me how difficult some user groups find scrolling as well. I had a guy grapple to click and hold the bottom arrow on the browser scroll bar for ages with a shaky hand (due to the heart medication he was taking). He wasn't aware that he could move down past the page fold in any other way.